Understanding KCL and the In=0 Principle in OMP Amplifiers

[Saeed.z]

i came across an example in my textbook as shown :

http://img593.imageshack.us/img593/8444/img1314i.jpg

if we used KCL and In = 0 :

In = I_100 + I_25

=> I_25 = -I_100

but as being written in the answer , I_25 = I_100 !

could anyone explain it for me ? thanks

 

[technician]

No current flows into or out of the + or - inputs of an ideal op amp.
Hope this helps

 

[sandy.bridge]

Wouldn't the technical answer be i_{25}=-i_{100} since the direction in the diagram is opposite to the actual direction? My textbooks take into account the assumed direction, and if the final answer is supposed to be opposite to the assumed direction, you merely place a minus sign in front of it to indicate that it is in the opposite direction to the supposed direction.

 

[technician]

Completely agree with Sandy Bridge